[llvm-bugs] [Bug 40776] New: missed optimization with C constructors of sparse structures

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40776

            Bug ID: 40776
           Summary: missed optimization with C constructors of sparse
                    structures
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arnd at linaro.org
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

I stumbled over a file with excessive stack usage in the Linux kernel when
building with clang, but not with gcc, reduced the test case to:

struct i2c_adapter {
  char name[48];
  int a[1000];
} hexium_attach_hexium;
void hexium_attach(void) {
    hexium_attach_hexium = (struct i2c_adapter){"hexium gemini"};
}

$ clang-8 -Werror  -c hexium-gemini.c -Wframe-larger-than=100  -O2
hexium-gemini.c:5:6: error: stack frame size of 4056 bytes in function
'hexium_attach' [-Werror,-Wframe-larger-than=]

Comparing the output shows that clang not only allocates stack space for the
extra copy of the structure, but also fails to optimize the string assignment:

https://godbolt.org/z/7jpy_y

I worked around it in the kernel sources by using an explict memset and
strscpy, but this seems like a generally useful optimization that could be
added.

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